Originally posted by sparky:
were's Paul from UK ?
Right here Sparky..... Been so busy in the other areas that I hadn't looked in here for a few days!
With the three different systems used here that I described some time ago, there's always been discussion over which is best.
All in all, the old urban system has a lot to recommend it. Neutral is grounded only at the utilty xfmr star-point, and the house ground is via the armor of the distribution cable, which is also grounded at the utility's sub-station. In the event of a ground fault, there's bound to be some current flowing through parallel earth paths, but the bulk of the fault current is confined to the cable armor. The main disadvantage is cost, which is why such systems haven't been installed here for years.
I take the point about possible effects of lightning where the neutral isn't grounded at the service entrance, but all of these systems were/are completely underground. (There's also the point that storms are nothing like as frequent or severe as in most parts of the U.S.A.)
To recap our PME system: The main panel still has separate neutral & ground busbars, but the ground is bonded to the neutral at the service block, which is immediately before the meter. Originally used only in areas where it was difficult to get a low loop impedance with a local ground rod, this system has now become widespread.
Because of the possible hazards of a broken neutral, our rules specify stricter bonding requirements for houses using PME.
In non-PME systems, the minimum size conductor for bonding to water/gas pipes etc. used to be 2.5 sq. mm (slightly larger than #14 AWG), although 4 sq. mm (a little over #12) is now standard.
Where PME is employed, all bonding conductors have to be min. 10 sq. mm (a little larger than #8).
The third system is probably the least satisfactory, for reasons I've alreay explained.
